Refrigeration systems used in household freezers, refrigerators, etc., are designed for low cost and high reliability, both of which require simplicity of design. Typical refrigerators or freezers employ a vapor compression system having an electric motor driven hermetic compressor connected in a circuit with a condenser, evaporator, and a refrigerant flow restriction of some sort between the condenser and the evaporator. In recent times energy conservation requirements have been imposed on these kinds of applicances. The conservation requirements have dictated a using a great deal of additional thermal insulation if system operating efficiencies remain the same as in the past. Adding such insulation would significantly reduce the size of the refrigerated compartment without any a reduction in the outside dimensions of the appliance. Design interest has consequently focussed on increasing refrigeration system operating efficiencies.
Conditions under which household freezers, etc., actually operate vary widely from theoretical design conditions. To accommodate varying conditions these appliances were constructed so that the compressor cycled on and off under control of a thermostat in the refrigerated compartment. When the thermostat was satisfied the compressor stopped. Refrigerant in the condenser continued to flow to the evaporator until the system pressure equalized. Pressure equalization usually occurred after all the liquified refrigerant passed from the condenser into the evaporator. Because the thermostat was satisfied before the refrigerant pressure equalization occurred, the cooling effect produced by the pressure equalizing flow was, in effect, wasted.
When the compressor restarted it was immediately required to reestablish the pressure differential across the system. Gaseous refrigerant thus had to be compressed and recondensed in the condenser for delivery to the evaporator before a significant cooling effect could recur. The pressure equalization flows and the ensuing refrigerant recompressions which such flows necessitated were inefficient.
Proposals were made to avoid inefficiencies created by pressure equalization flows. Some proposals involved using valves for blocking flow to the evaporator from the condenser whenever the compressor turned off. Some valves were solenoid operated and some responded to changes in refrigerant pressure created by the compressor turning on and off. For example, see U.S. Pat. No. 4,267,702 issued May 19, 1981 to Houk.
Pressure equalization flow blocking valves required the compressor to start against relatively high condenser back pressures resulting from the blocked equalization flow. Consequently it was important that the condenser outlet flow be unblocked promptly upon the compressor starting. Some valves were constructed to establish the condenser outlet flow in response to a rise in condenser outlet pressure created by the compressor start-up.
Another source of system inefficiency resulted from passage of hot gaseous refrigerant from the condenser outlet to the evaporator. There the gaseous refrigerant gave up heat to liquified refrigerant in the evaporator, thus reducing the cooling effect. Thus, it was desirable for the refrigerant flow from the condenser outlet to be restricted under conditions where hot gaseous refrigerant could pass to the evaporator (i.e. refrigerant temperature at the condenser outlet is high). Relatively unrestricted refrigerant flows were desirable when the condenser outlet temperatures were low (i.e. during subcooling).
Refrigerant flow modulating valves were proposed which operated in response to liquified refrigerant temperature at the condenser outlet. Some of these also blocked pressure equalization flows when the compressor turned off. See, e.g. U.S. Pat. No. 4,840,038. System efficiency could be improved not only by blocking pressure equalization flows but also by modulating the refrigerant flows to avoid inefficient operating conditions. Such a valve operated to restrict refrigerant flow when condenser outlet temperatures were relatively high and to permit unrestricted flow when condenser outlet temperatures were relatively low.
Owners of household freezers (and of some refrigerators) sometimes station the appliances out-of-doors or in unheated spaces. Where the applicance utilizes a refrigerant flow modulating valve which blocks pressure equalization flows when the compressor is off, problems can be encountered when the temperature ambient of the appliance is low. At low ambient temperatures, i.e. below 50.degree. F. and particularly well below freezing, the condenser temperatures can be so low that when the compressor starts operating it fails to create a sufficient pressure rise to open the valve. When ambient temperatures are low enough, the compressor can pump all the gaseous refrigerant from the evaporator into the condenser without increasing the condenser pressure enough to open the valve.
Failure of the appliance and loss of its contents becomes a distinct possibility in these circumstances. The flow controlling valve remains closed and therefore the thermostat can not be satisfied. The compressor thus operates unceasingly. In these appliances compressor lubricant is typically circulated with the system refrigerant. A likelihood of eventual compressor failure thus exists because of lack of lubrication. Compressor failure occurs unobtrusively and when the ambient temperature rises above freezing the contents of the appliance will eventually spoil.
The present invention provides a new and improved, highly efficient household refrigeration appliance wherein a refrigerant flow controlling valve is provided which controls the flow of liquified refrigerant through an expansion device in response to sensed condenser outlet refrigerant temperature, blocks refrigerant flow from the condenser under normal operating conditions when the compressor is off, yet is open and communicates the condenser outlet with the evaporator when the condenser outlet refrigerant temperature is below a predetermined level.